Abstract
SUMMARY Longitudinal data on an experimental subject arise when observations of a dependent variable are made at several successive values of an indexing variable x; often the indexing variable is time. When such data are recorded for a number of subjects, typically animal or human, the x-configuration may vary from subject to subject, producing irregular longitudinal data. Exact permutation methods are considered for comparing two or more groups of subjects. This approach entails choosing appropriate descriptive and test statistics and generating their exact distributions. The method is applied to two data sets, one relating to pigs and one to baboons. The second of these data sets involves a co-variate and the results are compared with those yielded by fitting a parametric mixed effects model. In the first example differences between groups were clearly established, and the graphical representations proposed were useful for describing the differences. In the second example the agreement between the results of the two methods of analysis was found to be very close.
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